Despite substantial progress over the last several decades, heart disease and its associated dysrhythmias remains one of the most prevalent causes of death in the world. In-hospital experience utilizing continuous cardiac monitoring revealed that prompt detection and diagnosis of cardiac dysrhythmias allows for rapid treatment and reversal of the cardiac dysrhythmia. Diligent physician and nursing intervention interrupts the natural progression of the patient's disease and increases the survival rate. This in-hospital experience is predicated upon trained personnel being able to recognize and correctly diagnose a patient's cardiac dysrhythmia and then provide the appropriate treatment.
Recent years have shown substantial progress in the development of automatic implantable cardioverter defibrillator systems or ICD's. These electronic standby cardioverter defibrillators will, in response to detection of abnormal cardiac rhythms, countershock the heart muscle via implanted electrodes with sufficient energy to depolarize the heart and mass. This countershock technique is directed at abolishing the origins of the pathologic dysrhythmia, thereby allowing the natural pacing activity of the heart to reestablish its dominance restoring the normal cardiac rhythm.
In general, several electronic standby defibrillator systems are disclosed in the prior art with examples such as U.S. Pat. Nos. 3,614,954 and 3,614,955. Other efforts in the field have resulted in the disclosure of implantable electrodes for use by automatic electronic cardioverter defibrillators, for example U.S. Pat. No. 4,030,509 issued to Hellman et al.
The one salient feature of all implantable cardiac devices is the necessity to detect electrical activity of the heart and form a diagnosis based on that detection. Typically, the first step in the detection process is to sense the electrocardiogram, amplify that signal, pass it through low pass and high pass filters, and from the resultant peaks generate a digitized signal representative of the electrocardiographic tracing. It is readily apparent that this digitized signal is easily detected, counted and measured for purposes of diagnostic paradigms in determining the presence or absence of cardiac arrhythmias. It then follows that the cardiac rate provides a simple threshold upon which to make a diagnosis of cardiac arrhythmia. If a preset heart rate threshold is surpassed, the diagnosis of cardiac arrhythmia is made and the appropriate treatment is carried out by the automatic ICD. Numerous systems incorporate heart rate as a detection parameter and incorporate the results of the detection within the diagnostic paradigms.
Simple straight forward application of a heart rate determination must be modified, however, in order to avoid an inadvertent defibrillation countershock in a person who may have sustained a rate of 250 beats per minute for a few beats. Such a patient may have simply sustained a premature ventricular contraction, or PVC. PVCs are abnormal contractions arriving soon after a normal ventricular contraction such that the interval measurement may equal or exceed the set heart rate threshold being monitored by the system. PVCs may not be benign by nature, but treatment for PVCs with electrical countershock therapy is inappropriate. The most common and simplest method of modification has been to additionally incorporate a duration parameter within the diagnostic paradigms such that an abnormally high heart rate must be sustained for a given number of beats before the automatic system will diagnose a cardiac arrhythmia and proceed with a countershock. Referring to FIG. 2, the X axis can be duration of tachycardia interval and the Y axis can be beats per minute. Thresholds are programmed within the system's diagnostic paradigm which derives a single point determination denoted here as X,Y such that if exceeded in both X and Y, as noted by the hash mark area, then a diagnosis of cardiac arrhythmia is made and treatment is carried out.
Other systems utilize different means of detection and threshold parameters. U.S. Pat. Nos. 4,184,493 and 4,202,340 both issued to Langer, disclose a system using a probability density function (PDF) to determine the presence or absence of ventricular fibrillation. Langer's system is such that a single point threshold is programmed and if the characteristic measured by the PDF is detected beyond the single point threshold, then a diagnosis of ventricular fibrillation will be made and countershock treatment would be carried out. Langer's system also disclosed a cardiac impedance measurement which was connected in a logical AND to the probability density function. If both events occur simultaneously, a single point threshold is reached and ventricular fibrillation would be diagnosed and countershock treatment carried out. While different than the traditional detection techniques, this system still fits the general characteristics of the graph in FIG. 2 where the X axis could be represented by the probability density function threshold and the Y axis could be presence of high or low impedance within the myocardium. As shown in FIG. 2, the single point (X,Y) is a logical AND connection of the X parameter and the Y parameter such that when both are satisfied then the determination falls within the hash mark area resulting in a diagnosis of fibrillation and subsequent treatment.
U.S. Pat. No. 4,475,551 issued to Langer, et al., again returns to the use of a probability density function approach to the detection of ventricular fibrillation. Langer recognized that the probability density function alone was insensitive and required modification. Langer coupled his probability density function in a logical AND with the heart rate, defining single thresholds for both. Again utilizing FIG. 2, this approach still defines a single point (X,Y) threshold determination beyond which satisfies the logical AND requirement. The end result continues to use the determination of a single point as a logical combination threshold that is either exceeded or rot.
There have been a number of patents describing various monitoring parameters. Examining these prior art disclosures reveal that almost all fall within the concept depicted in FIG. 2 such that one parameter can be placed on the abscissa, the second parameter on the ordinate, and single point thresholds satisfying the defined relationships are created. These thresholds are linked by a logical AND function whereby if both thresholds are exceeded, the point will fall within the hash mark area, diagnosis of cardiac arrhythmia is made and treatment is carried out.
U.S. Pat. No. 3,805,795 issued to Denniston and Davis discloses a system where the parameters monitored are myocardial contraction and heart rate. U.S. Pat. No. 4,796,620 issued to Imran discloses a system utilizing high cardiac contraction rate acceleration coupled with absence of a subsequent cardiac contraction rate deceleration linked in a logical AND to a high heart rate threshold. Imran attempts to avoid difficulties of confusing exercise induced tachycardia from pathologically induced ventricular tachycardia. U.S. Pat. No. 4,865,036 issued to Chirife disclosed two sets of parameters. The first set compared heart rate and a pre-ejection period. The second set of parameters in Chirife's disclosure measured the heart rate increase set against an absolute heart rate. U.S. Pat. No. 4,880,005 issued to Pless and Sweene disclosed a system which set four programmable detection criteria: high heart rate; rate stability; sudden onset; and sustained high rate. On the surface this may seem as a departure from the previous prior art disclosures but further analysis of Pless and Sweene's disclosure reveals that once again only single point thresholds are set for each of these four criteria and compared in a one-by-one fashion connected by logical ANDs. Other detection parameters have been mentioned within the prior art such as monitoring blood flow, pH, pCO.sub.2, pO.sub.2, arterial blood pressures, and body temperature. In all circumstances, a single point value threshold is set for each parameter. Through coupling by a logical AND to other parameters, a single point logical combinational threshold is set that is either exceeded or not.
The above mentioned detection systems have been developed in an effort to better delineate between the various ventricular arrhythmias. All of these attempts have been hobbled, however, because they are always limited to providing a single point threshold, ignoring the reality that none of these physiologic conditions relate to one another in such a fashion.
At the opposite end of the complexity scale are the neural networks with representative cases being the parent case of the present invention, U.S. patent application Ser. No. 07/837,952 entitled ARRHYTHMIA-DETECTION CRITERIA PERMITTING VARIATIONS IN INDIVIDUAL CARDIAC VARIABLES now U.S. Pat. No. 5,312,443 and U.S. Pat. No. 5,251,626 issued to Nickoils et al. entitled APPARATUS AND METHOD FOR THE DETECTION AND TREATMENT OF ARRHYTHMIAS USING A NEURAL NETWORK. In general, these detection systems disclose the use of neural networks or the use of complex mathematical functions to perform the detection and diagnosis for an ICD. While these type of systems have the potential for greater accuracy in detection and diagnosis of cardiac dysrhythmias, the problem is that neural networks and complex mathematical functions are overly complicated and difficult to program or train. In the case such as an ICD, where a neural network must make decisions that will ultimately determine whether a patient might live or die, such a device cannot afford the luxury of learning by its mistakes as is done to train most types of neural networks. Therefore, in order to teach a neural network sufficient information to perform its detection and diagnostic function effectively, the neural network must be derived prior to implantation of the device, something which is a difficult and time consuming process if customized to each individual patient.
Unfortunately, it is difficult to derive a neural network for effective detection and diagnosis that would be common to many patients because of the wide differences in patient conditions and needs. For example, the complexity required to program or train these systems effectively precludes an attending physician from altering the diagnostic parameters in response to information learned during the implantation procedure. Finally, the complexity of these types of diagnostic techniques necessarily increases the complexity and power consumption of the ICD which can lead to a potential increase in device failures as well as a decrease in device life span.
Any given patient with heart disease is at risk to develop a cardiac arrhythmia. In order to successfully treat the occurrence of an arrhythmia, an automatic implantable cardioverter defibrillator must contain within it the ability to distinguish accurately and consistently among the various ventricular tachycardias and provide the appropriate countershock technique to reverse the detected cardiac arrhythmia. Furthermore, any diagnostic paradigm must not only be able to distinguish among the various abnormal pathologic ventricular tachycardias but also distinguish the detected tachycardia from benign forms of ventricular tachycardia which do not require electrical intervention. Development of diagnostic paradigms to carry out this task have led to the development of the numerous above-mentioned parameters utilized by these detection systems in attempts to carry out appropriate delineation between the various tachycardias encountered. As noted, almost all of these systems choose a single programmable threshold level for each parameter and then connect it in a logical AND combination to a second parameter resulting in a single point logical combinational threshold, as universally depicted in FIG. 2. Only if both parameters are exceeded simultaneously can the system be confident it has detected an abnormal ventricular tachycardia and proceed with treatment.
The presence of so many diagnostic parameters merely indicates the severe limitation the single point logical combinational threshold paradigm places on existing diagnostic systems. While some of these diagnostic systems have recognized the limit of using only a single combinational threshold, these references teach that the way to overcome this limitation is to add more single point logical combinational thresholds on top of each other. Unfortunately, even these improved diagnostic systems still continue to have difficulty in reaching an appropriate diagnosis.
At the other end of the spectrum, other diagnostic systems have proposed the use of a complicated mathematical functions or neural network to interrelate a variety of cardiac detection parameters. While these systems may theoretically provide for a more accurate diagnosis, these systems are complicated to implement and also to program. What is needed is an improved programmable parameter system and method for more accurate interpretation and diagnosis of abnormal and benign ventricular tachycardias.